"Mobile devices that adapt to who you are, reflecting what you are doing when you are doing it. This concept is at the heart of the Plasma Active user experience. Plasma Active One was released in October 2011, providing early adopters the first opportunity to experience Activities on a tablet. Since then, the design and development team behind this open source touch interface has been hard at work on an update. The fruits of their labor were released today, December 14, 2011 as Plasma Active Two."

While I can see a benefit to activities, and I do try to use them myself, I fail miserably in adding them to my productivity:
1) if an activity is closed, and a program that was opened in that activity still resides in the system tray, opening that program will relaunch the activity that opened it: this is utterly wrong. Either the activity should remove it from the system tray on stopping the activity, or it should just open the window in the current activity. The result is a slow desktop that behaves badly and incoherently (coherence is one of their main thing of kde now isn't it?)
2) switching between activities is a pain in the ass, there used to be a alt+tab kindoff shortcut, but I can't find it anymore, and clicking every time on an icon to have a slow plasma (4-core system with 6gig of ram...) panel show up to click on a new icon just to switch to a different activity?
3) there's no visual meaning to activities, i.e. take webos or it's desktop counterpart gnome 3, a new activity has a meaning there, I know that in kde in each activity you can have different numbers of desktops, but how does a human being keep track of 16 different activities with each different numbers of desktops... rejoice the chaos created by order.
4) manually changing per window rule settings to tell a program it belongs to this or that activity, or even to multiple is just not productive and slow.
5) programs like opera do have a problem with this behavior, if you have two opera instances each in a different desktop, and you close one of these activities to later open them up again, guess what, opera will open a new window but not in the correct activity

Sure activities have potential, but after two+ years of development it's still not useful. The only reason I'm staying is because gnome 3 does not have the right programs, a task bar is useful (even with their desktop paradigm) and the (albeit slow) most functional window manager remains kwin.